The present invention is directed, in general, to wireless networks and, more specifically, to an adaptive digital pre-distortion correction circuit for use in an RF transmitter.
Every wireless network base station has an RF power amplifier for transmitting voice and/or data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, pagers, and the like) and a receiver for receiving voice and/or data signals from the mobile units. The design of an RF power amplifier (PA) for digital radio systems is controlled by two overriding criteria: 1) The RF power amplifier should transmit sufficient RF output power to serve the cell site of the base station in which it is installed, but should also use the minimum amount of input DC power in doing so; and 2) The adjacent channel power (ACP) noise (distortion) should be under certain limits (mask), that are usually defined in a standard (i.e., ACP profile)
In most cases, these two criteria are contradictory. ACP noise results from non-linear effects, such as over-driving the power amplifier into its non-linear region (clipping). Spurious spectral components are introduced when a signal peak is sufficiently large to over-drive or to saturate an RF amplifier in the transmitter. In order to meet the ACP profile, the RF transmitters in wireless networks in which digital signals have high peak-to-mean ratios, such as CDMA and multi-carrier systems, are frequently xe2x80x9cbacked offxe2x80x9d from full power (or peak power) to avoid operating the transmitter in non-linear conditions. In these digital systems that have high peak-to-mean signal ratios, the RF power amplifier thus needs a considerable amount of power xe2x80x9cheadroomxe2x80x9d to accommodate the peak power. For example, RF power amplifiers in some CDMA systems need more than 10dB of headroom space to protect the peak CDMA signal power from clipping. Unfortunately, leaving this much overhead significantly reduces the power efficiency of the RF power amplifier. This increases the DC power consumption, the base transceiver subsystem cooling requirements, the overall system volume, weight, and cost.
For a particular digital radio system, such as cellular CDMA or TDMA, the transmitter ACP profile is defined in the system standard. Generally speaking, the actual ACP profile of an RF power amplifier is not the same as the ACP profile required by the standard. The power amplifier ACP profile is determined more or less by the power amplifier device characteristics, operating modes, and signal behaviors. For example, the out-of-band spurious components generated from a CDMA signal appear like white noise: the power density does not change significantly with frequency. However, the ACP profile defined in, for example, the IS95 CDMA system standard does not require a constant spurious power density over different frequencies. The whole frequency spectrum is divided into a few blocks and the standard ACP profile changes significantly from one block to the next.
This may lead to situations in which the power amplifier output power level is dictated by the ACP noise at a few frequency points where the standard ACP profile appears the most stringent. However, there may still be relatively large ACP noise margins at many other frequencies. In a sense, the power amplifier ACP noise is not optimized to make full usage of the ACP profile under the applicable standard. The excess ACP noise margin at most frequencies is not used.
Prior art solutions for allowing RF power amplifiers to operate more closely to full power in systems having high peak-to-mean digital signal ratios typically use a digital pre-distortion adjustment circuit that uses the input signal, the output signal, and the standard ACP profile to optimize the performance of the RF amplifier to more closely match the desired standard ACP profile. These conventional digital pre-distortion methods sample, digitize, synchronize, and compare input and output signals to determine the signal distortion. The amount of correction is usually based on the difference between the input and output signals.
However, comparison of input and output signals requires sophisticated circuits for synchronizing the signals over time and temperature in order to extract correct signal distortion information. In addition, the pre-distortion correction step, which is based upon the difference between the input and output signals, may not yield the optimum correction in terms of efficiency, speed, and amount of required circuitry.
There is therefore a need in the art for improved wireless networks that use more efficient RF power amplifiers. In particular, there is a need for improved RF power amplifiers that can operate more closely to full power in systems having high peak-to-mean digital signal ratios. More particularly, there is a need for power control apparatuses that make RF power amplifiers more efficient by utilizing the available ACP noise margins under the applicable standard ACP profile. There is a further need for power control apparatuses that are not limited by circuitry required to synchronize the RF input and RF output signals in order to generate pre-distortion correction signals.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a pre-distortion correction circuit for use in an RF transmitter having a transmit path capable of receiving and amplifying an RF input signal. The pre-distortion correction circuit modifies distortion in the RF output signal caused by an RF power amplifier in the transmit path. In an advantageous embodiment of the present invention, the pre-distortion circuit comprises: 1) feedback circuitry coupled to an output of the transmit path capable of demodulating and digitizing the distorted RF output signal to thereby produce a first demodulated digital output signal; and 2) a pre-distortion calculation controller coupled to the feedback circuitry capable of comparing the first demodulated digital output signal to adjacent channel power (ACP) profile data associated with the pre-distortion calculation controller and generating therefrom pre-distortion correction values capable of being used to predistort the RF input signal to thereby cause the RF output signal to more closely resemble an ideal RF output signal within the limits of the ACP profile.
Advantageously, no synchronization circuitry is needed on either the input of the output in order to pre-distort the RF input signal. Instead, the pre-distortion correction values used to generate an ideal RF output signal are derived solely from the actual RF output signal and the applicable ACP profile limits.
According to one embodiment of the present invention, the transmit path comprises demodulation circuitry capable of demodulating and digitizing the RF input signal to thereby produce a first demodulated digital input signal.
According to another embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first demodulated digital input signal produced from the input RF signal.
According to an intermediate frequency (IF) embodiment of the present invention, the feedback circuitry comprises a first intermediate frequency (IF) demodulator and the first demodulated digital output signal comprises a first digital IF output signal.
Still according to the IF embodiment of the present invention, the demodulation circuitry in the transmit path comprises a second intermediate frequency (IF) demodulator and the first demodulated digital input signal comprises a first digital IF input signal.
Further according to the IF embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first digital IF input signal produced from the input RF signal.
According to a baseband embodiment of the present invention, the feedback circuitry comprises a first baseband demodulator and the first demodulated digital output signal comprises a first digital baseband output signal.
Still according to the baseband embodiment of the present invention, the demodulation circuitry in the transmit path comprises a second baseband demodulator and the first demodulated digital input signal comprises a first digital baseband input signal.
Further according to the baseband embodiment of the present invention, the pre-distortion correction values are used to pre-distort the first digital baseband signal produced from the input RF signal.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.